Conventionally, to determine the angle of optical rotation, i.e., the angle through which the plane of polarization is rotated in a sample, linearly polarized light is directed into the sample and the beam of light passed through the sample is input via an analyzer to a photodiode for conversion into an electrical signal. The angle of rotation is determined from the thus obtained signal. When the tilt angle of the transmission axis of the analyzer with respect to the transmission axis of a polarizer is denoted by θ, and the angle of rotation produced by the sample is denoted by α, the intensity of light, I, received by the photodiode is given as I=T×I0 cos(θ−α)2. Here, T represents the transmittance considering all attenuations due to reflections and absorptions in the sample, the polarizer, and the analyzer, and I0 designates the intensity of the incident light. As can be seen from the above equation, as the analyzer is rotated, a minimum point is obtained for every rotation angle π (rad). The angle of optical rotation can be obtained from the angle of the analyzer at this minimum point. For increased accuracy and sensitivity, a polarization plane vibration method is generally used.
This will be explained below with reference to FIG. 5. Monochromatic light emitted from a light source 121 enters a polarizer 122 which is driven by a polarizer driving circuit 129 and vibrating with a frequency f and an angular amplitude θ, and the light is converted into linearly polarized light whose plane of polarization is vibrating in a rotating fashion. When this beam of light is introduced into a sample 125 and passed through an analyzer, a signal of frequency f is obtained from a photodiode 124. Assume here that the plane of polarization is rotated through a by the optical activity of the sample 125; then, when the polarizer 122 and the analyzer 123 are arranged at right angles to each other, a signal with an inverted phase is obtained, depending on whether the optical activity of the sample 125 is right-handed or left-handed. The signal obtained from the photodiode 124 is amplified by an amplifier circuit 126 and filtered and rectified by a filtering/rectifying circuit 127 to obtain the phase. Then, the angle of the analyzer is determined in accordance with the optical null method by rotating the polarizer 123 by means of an analyzer driving circuit 128 in the normal or reverse direction according to the obtained phase so that the amount of light transmission becomes a minimum. A signal of double frequency 2f is obtained at an equilibrium point, but the 2f component is removed by synchronous detection. The angle of the analyzer at the equilibrium point corresponds to the angle of optical rotation produced by the sample. As a method of rotating the plane of polarization, the polarizer is mechanically rotated, or alternatively, a Faraday rotator is used.
The above method, however, requires that the polarization state of light be preserved to a certain extent by transmission through the sample, and therefore it can only measure transparent or near-transparent samples. The resulting disadvantage is that, in the case of a sample such as a scattering sample, that changes the polarization state to random polarization, the S/N ratio degrades and the angle of optical rotation cannot be measured. Another disadvantage is that, as the polarizer must be mechanically rotated, or a Faraday rotator must be used, to rotate the plane of polarization, the device becomes complex and the price and size increase.